The present invention is directed generally to the transmission of signals in optical communications systems. More particularly, the invention relates to systems, devices, and methods for producing and transmitting modulated optical signals having different polarization orientations.
The development of digital technology provided the ability to store and process vast amounts of information. While this development greatly increased information processing capabilities, it was soon recognized that in order to make effective use of information resources it was necessary to interconnect and allow communication between information resources. Efficient access to information resources requires the continued development of information transmission systems to facilitate the sharing of information between resources. One effort to achieve higher transmission capacities has focused on the development of optical transmission systems. Optical transmission systems can provide high capacity, low cost, low error rate transmission of information over long distances.
The transmission of information over optical systems is typically performed by imparting the information in some manner onto an optical signal. In most optical transmission systems the information is imparted by using an electrical data stream either to directly modulate an optical source or to externally modulate an optical carrier so that the information is carried at the frequency of the optical carrier, or to modulate the information onto one or more subcarriers or sidebands, with the later technique sometimes called sub-carrier modulation (“SCM”). There are many variations of modulators. Examples of dual path modulators are described in U.S. Pat. No. 5,745,273.
SCM techniques, such as those described in U.S. Pat. Nos. 4,989,200, 5,432,632, and 5,596,436, generally produce a modulated optical signal in the form of two mirror image sidebands at wavelengths symmetrically disposed around the carrier wavelength. Generally, only one of the mirror images is required to carry the signal and the other image is a source of signal noise that also consumes wavelength bandwidth that would normally be available to carry information. Similarly, the carrier wavelength, which does not carry information in an SCM system, can be a source of noise that interferes with the sub-carrier signal. Modified SCM techniques have been developed to eliminate one of the mirror images and/or the carrier wavelength, such as described in U.S. Pat. Nos. 5,101,450 and 5,301,058.
Initially, modulated optical signals were spatially separated by placing each optical signal on a different fiber to provide space division multiplexing (“SDM”) of the information in optical systems. As the demand for capacity grew, increasing numbers of information data streams were spaced in time, or time division multiplexed (“TDM”), on the single optical signal in the SDM system as a means to better use the available bandwidth. The continued growth in demand has spawned the use of wavelength division multiplexing (“WDM”) to transport multiple optical signals on a single fiber. In WDM systems, further increases in transmission capacity can be achieved not only by increasing the transmission rate of the information on each wavelength, but also by increasing the number of wavelengths, or channels, in the system.
There are two general options for increasing the channel count in WDM systems. The first option is to widen the transmission bandwidth to add more channels at present channel spacings. The second option is to decrease the spacing between the channels to provide a greater number of channels within a given transmission bandwidth. The first option currently provides only limited benefit because most optical systems use erbium doped fiber amplifiers (“EDFAs”) to amplify the optical signal during transmission, and EDFAs have a limited bandwidth of operation and suffer from non-linear amplifier characteristics within the bandwidth. Difficulties with the second option include controlling the wavelengths of WDM optical signals and/or controlling the optical sources to prevent interference from wavelength drift and nonlinear interactions between the signals.
A further difficulty in WDM systems is that chromatic dispersion, which results from differences in the speed at which different wavelengths travel in optical fiber, can also degrade the optical signal. Chromatic dispersion is typically controlled using one or more of three techniques. One technique is to offset the dispersion of the different wavelengths in the transmission fiber through the use of optical components such as Bragg gratings or arrayed waveguides that vary the relative optical paths of the wavelengths. Another technique is to intersperse different types of fibers that have opposite dispersion characteristics to that of the transmission fiber. A third technique is to attempt to offset the dispersion by prechirping the frequency or modulating the phase of the carrier source in addition to modulating the data onto the carrier. For example, see U.S. Pat. Nos. 5,555,118, 5,778,128, 5,781,673 or 5,787,211. These techniques require that additional components be added to the system and/or the use of specialty optical fiber that has to be specifically tailored to each length of transmission fiber in the system.
New fiber designs have been developed that substantially reduce the chromatic dispersion of WDM signals during transmission in the 1550 nm wavelength range, such as dispersion shifted fiber and non-zero dispersion shifted fiber. However, the decreased dispersion of the optical signal allows for increased nonlinear interaction between channels, such as four wave mixing, which increases signal degradation. The effect of lower dispersion on nonlinear signal degradation becomes more pronounced at increased transmission rates due to the higher signal launch power used at higher transmission rates.
Non-linear interactions can be reduced if adjacent data signals are linearly polarized and oriented orthogonal to each other. For example, see U.S. Pat. No. 5,111,322, issued on May 5, 1992. Such systems, however, still have certain drawbacks, such as requiring two modulators to produce a pair of orthogonal signals. As a result, the size, cost, and power consumption of such systems will increase significantly as the number of WDM channels increase. Accordingly, there is a need to reduce the number of components in optical systems, particularly expensive components such as modulators, while at the same time reducing the effects of phenomena, such as chromatic dispersion and non-linear interactions.
The many difficulties associated with increasing the number of wavelength channels in WDM systems, as well as increasing the transmission bit rate have slowed the continued advance in communications transmission capacity. In view of these difficulties, there is a clear need for transmission techniques and systems that provide for higher capacity, lower cost, longer distance optical communication systems, devices, and methods.